Its worth it at 10 bucks just because there are very few games out there with that style of play. Oldschool twitch gaming is almost dead, enjoy it while you can.
Yea, it's not all that great, I started playing it right after I beat Shinobi and was kinda disappointed, I wouldn't say it's terrible, but it's nothing really special either.
If you like the gameplay of Shinobi, do check out Nightshade. If you liked anything else about Shinobi - the atmosphere, style, music, presentation, etc. - then you may run into difficulties enjoying Nightshade.
Biggest problem i had with the game was they took out the option to leave the original japanese diaglogue. Minor thing really but after playing through most of shinobi with the japanese diaglogue the english track for nightshade was damn near unbearable heh.